1. Field of the Invention
This invention generally relates to an organic light emitting diode display device, and more particularly to an organic light emitting diode display device having a light-absorbing layer.
2. Description of Related Art
Flat panels haven been widely used in communication industries as interfaces between the users and the machines such as portable communication products. Some major flat panel technologies are plasma display, liquid crystal display, electro-luminescent display, light emitting display and electro-chromic display. However, compared to those flat panel technologies, organic light emitting diode (“OLED”) has advantages of self-emitting, less power consumption, low costs, low temperature operation, high response speed, and no viewing angle dependence. Hence, it is highly potential that OLED would be the flat panel display for next generation.
FIG. 1 is the cross-sectional view of a conventional organic light emitting diode display device. The conventional organic light emitting diode display device includes a transparent substrate 100, a plurality of strip anodes 102, an organic functional layer 104 and a plurality of strip cathodes 106. The transparent substrate 100 usually is a glass substrate; the transparent anode 102 material usually is Indium-Tin-Oxide (“ITO)”; the organic functional layer 104 is a multi-layer organic thin film, wherein the organic functional layer 104 includes the hole injecting layer, the hole transmitting layer, organic light emitting diode, the electron transmitting layer, and the electron injecting layer; the metal cathode 106 material usually is Al, Ca, or Mg—Ag alloy.
The ratio of the maximum brightness and the minimum brightness is an important factor to decide the quality of a display device. This ration is so called “contrast ratio” (“CR”). The higher the ratio, the better the display device. The definition of the contrast ratio is as follows:                     CR        =                                            L                              sub                ,                on                                      +                          R              amb                                                          L                              sub                ,                off                                      +                          R              amb                                                          (        1        )            
Wherein Lsub, on is the brightness when the pixels are turned on; Lsub, on is the brightness when the pixels are turned off; Ramb is the brightness of the reflected light when the incident light emits into the display device. Assume that the brightness when the pixels are turned on is 100 and the brightness when the pixels are turned off is 1. Hence, the relationship between CR and Ramb can be shown according to (1).
FIG. 2 shows the relationship between the contrast ratio and the brightness of the reflected light for a conventional organic light emitting diode display device. As shown in FIG. 2, the higher the brightness of the reflected light, the less the contrast ratio. In other words, the recognition ability of a conventional organic light emitting diode display device becomes worse when the incident light is too strong. Hence, how to increase the recognition ability of the organic light emitting diode display device becomes an important concern.
LUXELL proposes an organic light emitting diode display device having an optical interference layer by adding a thin metal quasi-transmitting layer and a transparent material layer between the organic emitting layer and the metal cathode. Because of the effect of the destructive interference, the reflection rate can be reduced to below 1% in order to increase the contrast ration of the display device.
However, the organic functional layer of the organic light emitting diode display device is formed by evaporation. The transparent material layer (metal oxide) of the optical interference layer is formed by sputtering. The major drawbacks of the above structure are as follows: 1. It requires different equipment to form the organic functional layer and the transparent material layer. 2. The stress mismatch when using different equipment to form the organic functional layer and the transparent material layer would cause a higher err rate. 3. The organic functional layer would be damaged by the ion bombardment when forming the optical interference layer. 4. Sputtering process increases the costs for forming the optical interference layer because of higher costs of the sputtering equipment.